The invention relates to a synchronizing ring for a synchronizing apparatus of a switchable gear changing transmission.
Synchronizing rings in a mechanically switchable gear changing transmission, for example in vehicle gearboxes, serve to compensate relative speeds occurring during a gear change between a gear wheel and a gear shaft. In this connection the synchronization is achieved by friction between the corresponding friction partners. The method of operation of gears of this kind and the operating sequence of the synchronization process are known per se and do not need to be explained more closely to the person averagely skilled in the art.
For protection against premature wear and/or for the improvement of the friction characteristics it is known to provide the friction surfaces of synchronizing rings which are, as a rule, manufactured from a metal or a metal alloy, such as from brass or steel for example, with a friction layer. In this connection, completely different types of friction layers are in use, for example thermal spray layers made of molybdenum, carbon friction layers or friction layers made of other materials.
However, not only are ever increasing demands made on the characteristics of the friction layers by the ever increasing loads which act on the synchronizing rings in the operating state, but also the strength and stiffness of the synchronizing rings per se, i.e. of the synchronizing ring body, are increasingly the subject of current developments in transmission design.
A synchronizing ring is disclosed in DE 197 13 304 B4 for example, which is formed without cutting from a thin-walled, through hardenable, steel band, with means for stiffening being provided at a contact surface or at regions of the synchronizing ring projecting from the friction surface while retaining the load carrying part of the friction surface.
In this connection the means for stiffening in accordance with DE 19 13 304 B4 can be realized by means of different measures. For example, a solid rim which starts from one edge of the friction surface and forms a rim directed radially to the inside or a free end of the synchronizing ring can be provided at the end with a coined feature or upset portion which increases the wall thickness.
Although an increase in the solidity and stiffness of the synchronizing ring can be achieved through the measures discussed in DE 197 13 304 B4, all the solutions shown there have a decisive disadvantage which results in particular from the fact that the retention of the load carrying part of the friction surface is absolutely demanded as a central feature of these synchronizing rings, regardless of how or by using what measures the stiffening of the synchronizing ring is to be achieved.
In this kind of synchronizing ring it is to be feared that in the operating state, in particular under high loads and/or if very fast gear changes of the transmission are required, an only inadequate lubrication takes place between the contact surface of the synchronizing ring and its counterpart. This is because, among other things, during the synchronizing process, too much oil is displaced in the radial direction by axial pressure on the contact surface of the synchronizing ring, so that the contact surface, which rotates with a certain relative speed relative to its counterpart, stands in friction contact with its counterpart more or less without lubrication, which leads to premature signs of wear on the synchronizing ring and its counterpart and, in the worst case with heavy loads, to spontaneous damage to the corresponding surfaces.
A carrier for a cone-shaped synchronizing ring is described in DE 35 19811 A1 which has an outer gear ring divided into thirds and which is arranged in a cylindrical cavity of a synchronizing ring hub, which is often referred to as a synchronizing ring body and is substantially rotationally fixedly anchored to the synchronizing ring hub via abutments which are formed as lugs. “Substantially rotationally fixedly” means in this connection, within the context of this application, that the synchronizing ring is rotationally fixedly connected to the synchronizing ring hub apart from slight angular movements in the peripheral direction of the synchronizing ring.
DE 198 53 856 A1 describes a continuation of the above-described synchronizing ring, wherein the synchronizing ring of DE 198 53 856 A1 is characterized in that its width is substantially only determined by the required width of its friction surface. This is achieved by the fact that an abutment which corresponds with regard to its function to the lugs in accordance with DE 35 19811 A1 is arranged at or near to an end section of the ring body with a smaller cone diameter, and also the contour of the abutment projects in the radial direction beyond the contour of the external jacket surface of the ring body.
Both synchronizing rings are in principle well secured in the operating state against relative rotation with regard to the synchronizing ring hub by the lugs or abutments, however they both lack a reliable guide in the radial direction in the cylindrical cavity in the synchronizing ring hub.
This means that the synchronizing rings known from the prior art are normally secured against a rotation in the peripheral direction in relation to the synchronizing ring hub, however these rings have an increased tendency towards uncontrolled movements, e.g. to small radial excursions or tilting movements due to their conical outer shape, which cooperates with a cylindrical inner contact surface in the cylindrical cavity of the synchronizing hub. These uncontrolled movements can express themselves for example in unpleasant vibrations, thus exerting a negative influence on the reliability and precision of the synchronizing process. This can lead to an increase in the switching times, to faster and increased wear of the friction surface and of the entire synchronizing ring and thus to shorter repair and servicing intervals, quite apart from the fact that the ease of gear changing and the driving comfort of a heavy goods vehicle is considerably limited by the poor guidance of the synchronizing ring in the cavity of the synchronizing ring hub and the uncontrolled movements. These effects become all the more significant the more power or torque has to be switched by a corresponding transmission.
In a simple case a synchronizing ring set comprises a synchronizing ring and an inner ring and the inner ring can be constructed in metal, in brass, in a sintered metal, in a plastic, in a composite material or in any other suitable material. In order to improve the friction with a friction partner the inner ring can be coated or covered with a friction coating, for example with a carbon layer. In the synchronizing ring itself, which has an outer toothed ring, at least in sections, recesses are provided in a manner known per se, into which lugs of the inner ring can engage, so that the inner ring is rotationally fixedly connected to the synchronizing ring.
It is important in this connection that the inner ring is, in no operating state, significantly displaced in the axial direction relative to the synchronizing ring, since in such a case the lug of the inner ring can slip out of the recesses of the synchronizing ring so that the inner ring is no longer secured against rotation relative to the synchronizing ring, which can lead to destruction or at least to considerable damage to the transmission.
If a synchronizing ring set is in operation for a long period of time, it goes without saying that it is subject to a certain degree of wear which leads to a mechanical erosion of the components of the synchronizing ring set, for example by wear of the friction coating, in other words, for example, of the carbon friction layer, so that pre-set tolerances can no longer be maintained, making it ultimately possible that the lug of the inner ring jumps out of the recess in the synchronizing ring, leading to the above-mentioned damage to the transmission.
Thus, the tolerances of the individual components, which underlie the design of the synchronizing ring set and which arise in the course of time, have to be taken into consideration. For this reason one attempts to increase the length of the lugs of the inner ring more than would actually be necessary for the function of the inner ring (namely the holding of the inner ring in the synchronizing ring) without tolerance considerations for example due to wear being necessary.
These lugs cannot be selected to be of any length however, because otherwise a collision with the components behind the synchronizing ring (synchronizing ring body) could result.
A concept for resolving this is to provide bulges or raised portions in the synchronizing ring at the level of the recesses for the lugs of the inner ring, for example in the form of thickened portions. By this means shorter lugs would also be held securely. The disadvantage of this is that the wear reserve, which is generally known to the person averagely skilled in the art as the space between the inner ring and the synchronizing ring in the axial direction, is reduced in this way, which is also not desired. A solution for this problem would be to reduce the height of the inner ring between the lugs. A consequence would then be the loss of friction surface on the inner ring which likewise does not lead to the desired result.